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SYNTHESIS AND THE STUDY OF MAGNETIC CHARACTERISTICS OF NANO Lai_xSrxFeO3 BY CO-PRECIPITATION METHOD
A. T. Nguyen1, M.V. Knurova2, T. M. Nguyen3, V. O. Mittova4, I.Ya. Mittova5 :Ho Chi Minh City Pedagogical University, Ho Chi Minh City, Viet Nam 2Voronezh State University, Voronezh, Russia 3Ha Noi National University of Education, Ha Noi, Viet Nam 4Voronezh State Medical Academy, Voronezh, Russia 5Voronezh State University, Voronezh, Russia
anhtien0601@rambler.ru, cnurova2010@yandex.ru, montvn@gmail.com, vmittova@mail.ru, imittova@mail.ru
PACS 75.50.Cc, 81.07.Wx
The goal of this study was the sol-gel synthesis of nanocrystals Lai_xSrxFeO3 (x=0.0, 0.1, 0.2, 0.3) and an examination of their magnetic properties. An aqueous solution of ammonia and 5% ammonium carbonate solution were used as precipitating agents. It was established that the crystallization of LaFeO3 is completed at 750° C (annealing for 1 h). The average diameter of the synthesized particles was 80-100 nm. Investigation of the magnetic properties showed non-monotonic changes of saturation magnetic moment and increase of coercive force with increased Sr content in the sample.
Keywords: sol-gel synthesis; nanopowders; lanthanum ferrite; magnetic properties; high-coercivity materials.
Received: 28 August 2014 Revised: 13 October 2014
1. Introduction
In the late 1980's, nanotechnology began to develop and achieved many great improvements not only in research but also in many applications. Nanomaterials and nanotools exhibit new physical and chemical effects and properties that do not exist in bulk materials with the same chemical composition [1].
Materials widely used in practice are magnetic materials and other functional materials, which are applied to various electronic devices such as transformers, generators, electric motors, digital detectors, voice recorder, video recorder, etc. These materials, having ABO3-typed perovskite-like structure (where A is a such as Y, La, Ln (rare earth metals), Bi and B is transition metal such as Mn, Fe, Co , Ni, Cr), are studied intensively due to their outstanding properties and difficult synthesis [5-11]. These materials are also studied as catalysts [12].
Modified ABO3 compounds are materials where A or B or both A and B ions are partially replaced by other metal ions (such as Ca, Sr, Cd, Ln (rare earth metals), Mn, Fe, Ni, Al, etc.) [13-17]. This modification produces mixed-valence state of metals and structural defects, which create more interesting effects such as thermal effects, thermomagnetic effects, and large magnetoresistance for material substrate. That has opened up new application of
perovskite materials in a number of modern industrial areas such as electronics, information, petro-chemical processing technologies.
Currently, in order to prepare ABO3-typed perovskite materials with small particle size, various basic methods are commonly used, such as co-precipitation at room temperature, sol-gel method, co-complexing method, etc. The advantage of these methods is that the crystallization process occurs at lower temperatures than that of traditional ceramic synthesis and the obtained composite materials have more uniform compositions with higher purity. However, to synthesize nanosized ABO3 materials via these methods, it is required to investigate many factors affecting the formation of the single-crystalline phase such as temperature, calcination time, pH, gelling substance/metal molar ratio, gelling temperature, etc. This investigation requires much time and effort.
Currently, much attention is being devoted to the investigation of the particle size, structure and properties of substituted ferrite. In particular, [15] it was shown that the doping of yttrium ferrite by calcium leads to a reduction of particle size from 50 nm to 27.5 nm, an increase of magnetization (J) from 0.070 to 0.148 Am2/kg and a decrease in the coercive force (Hc) from 3.58 to 2.78 kA/m in a field of 640 kA/m. The fact that the doping of cobalt ferrite with zinc leads to increased magnetization from 71.297 Am2/kg for CoFe2O4 up to 152.531 Am2/kg for Co0.5Zn0.5Fe2O4 was demonstrated [18].
However, the synthesis of nano Lai_xSrxFeO3-typed perovskite materials by hydrolysis of metal cations in boiling water and the addition of a solution containing a precipitating agent and the study of magnetic characteristics of the synthesized material have not been reported.
In this study, nano La1-xSrxFeO3-typed perovskite materials (x = 0.0, 0.1, 0.2 and 0.3) were synthesized using the co-precipitation method in boiling water and studied their magnetic properties.
2. Materials and methods
The chemicals used in the research were analytically pure: Fe(NO3)3-9H2O, La(NO3)3-6H2O, Sr(NO3)2, 25% NH3, (NH4)2CO3. The salts were dissolved in water with the concentration of 0.15M. Precipitating agents used in the experiments were aqueous ammonia and ammonium carbonate in 15% excess as compared to the amount in stoichiometry equation to ensure the complete precipitation of metal ions in the solution.
Many studies demonstrate that sols of iron and lanthanum oxides, and a mixture of sols of iron (III) and lanthanum oxides can be obtained by co-precipitation of Fe3+ and La3+ in boiling water [11, 19, 20]. The results of a previous study [11] demonstrated that use of the co-precipitation is the optimal method.
Nano La1-xSrxFeO3 materials were synthesized by adding an aqueous solution containing La(NO3)3, Sr(NO3)2 and Fe(NO3)3 drop wise into boiling water with the molar ratio La : Sr : Fe = (1- x) : x : 1 and x = 0.0, 0.1, 0.2, 0.3 with magnetic stirring. The solution was boiled for 5 minutes and then cooled down to room temperature. Then, 5% ammonia solution (for x = 0.0) and 5% ammonium carbonate (x = 0.1, 0.2, 0.3) were slowly added into the obtained solution with concurrent vigorous stirring for about 30 minutes. The obtained precipitate was separated using vacuum filter, washed with distilled water before being dried naturally at room temperature. The obtained powder was finely ground then calcined in air at different temperatures: 750, 850 and 950°C to examine the completion of crystallization process and the formation of homogeneous phase.
The physicochemical processes, occurring when heating the samples, were studied by DTA/TGA method performed on a TGA Q500 V20.13 Build 39, with a maximum temperature of 1100°C and heating rate of 10°C/min.
The phase composition of obtained powder was examined by X-ray diffraction method on a D8-ADVANCE.
Elemental composition of the samples was tested by the method of energy-dispersive X-ray spectroscopy (EDXS, INCA Energy-250) and atomic absorption spectroscopy according to the reference of Ref. AOAC 965.09 (ICE-3500).
The size and shape of particles were determined by scanning electron microscope (SEM, JE-1400).
The magnetic characteristics (saturation magnetization, remanent magnetization, coercive force) of the material were determined on vibrating sample magnetometers Microsene EV11, performed at room temperature.
3. Results and discussion
3.1. LaFeO3 nano-materials synthesis results
Fig. 1 shows TGA diagram of powder sample (with x = 0.0) dried naturally at room temperature.
Fig. 1. TGA/DTA diagram of powder sample
It can be seen from Fig. 1 that the TGA curve exhibited 3 weight loss effects with a total weight loss of about 40%. The first weight loss period occurred from room temperature to 370°C with the weight loss of 23.67% corresponding to an endothermic peak at 92.20°C and a small peak at 42.21°C, which was attributed to the loss of surface water, desorption, and the dehydration of iron(III) hydroxide to from FeOOH [11]. The second weight loss period occurred from 370°C to 650°C, the third period started at 650°C and finished at 750°C corresponding to the weight loss of 11.051% and 5.197%, respectively. This was due to the heat absorption of complete decomposition of FeOOH and La(OH)3 to form corresponding oxides. From about 750°C, it could be observed that the weight of sample was unchanged with respect to the formation of single LaFeO3 phase. It is notable that the exothermic
effect for the phase transformation of LaFeO3, which forms the corresponding oxides, was not observed. This can be explained by the thermal compensation occurring when LaFeO3 transformed from amorphous to crystalline nano-phase. A similar phenomenon was described in [17, 19].
From the results of TGA analysis, the calcination temperatures of 750, 850 and 950°C were chosen to study the formation of single phase LaFeO3. Fig. 2 shows the XRD LaFeO3 pattern of samples after calcination at 750, 850 and 950°C for 1 h.
* La^Oj
2-Tb?t3 -Swle
Fig. 2. XRD pattern of LaFeO3 sample heated for 1 h
In the XRD pattern, the typical peaks of single phase of crystalline perovskite LaFeO3 were observed. It can be inferred that the crystallization is almost completed at 750°C, there were only 2 peaks with low intensity at 750°C. At 850°C and 950°C peaks of impurity were not observed.
The result of scanning electron microscope (SEM) of the samples after calcination at 850°C (t = 1 h) shows that the particles exhibit spherical or oval shapes (Fig. 3) with two ranges of size: small particles with a size from 50 to 70 nm, and large particles with a size of about 100 nm or larger. Additionally, it is also demonstrated in the SEM images that the particles bound together to form bulks of particles. This is the limitation of synthesized metal oxides, and particularly LaFeO3, using the co-precipitation method.
As a result, the LaFeO3 material with a particle size of 100 nm was synthesized via the co-precipitation of metal cations Fe3+ and La3+ in boiling water with aqueous ammonia as co-precipitating agent and calcination temperature of 850°C.
3.2. Lai_xSrxFeO3 nano-materials synthesis results (x=0.1; 0.2; 0.3)
Because of the high solubility product of Sr(OH)2 (Ts = 3.2x10-4 at 20°C) [20], and the small solubility product of Fe(OH)3 and La(OH)3 at the same temperature (3.8x10-38 and 1.05x10-21 respectively) [20], it was not possible to co-precipitate La3+, Sr2+ and Fe3+ cations by aqueous ammonia. Therefore, co-precipitating agent was replaced by ammonium carbonate. The results obtained from experiments of La1-xSrxFeO3 (x = 0.1, 0.2, 0.3) synthesis are presented in the section 2.
Fig. 3. SEM picture of LaFeO3 sample heated at 850°C (t=1 h)
From the TGA results of synthesized LaFeO3 powders (see Fig. 1), we chose samples with calcination temperatures at 750°C, 850°C and 950°C for 1 h to investigate the formation of single La1-xSrxFeO3 phase.
Figure 4, 5, 6 present the XRD patterns of La1-xSrxFeO3 materials (x = 0.1, 0.2, 0.3) after calcination at temperatures of 750°C, 850°C and 950°C for 1 h. Figures 4, 5, 6 shows that the obtained products had composition of orthorhombic LaFeO3 phase.
However, the lattice spacing (d) of synthesized crystalline samples decreased with increasing x values (table 1). This could be explained by the replacement of La3+ ions by Sr2+ ions that led to the lattice shrinkage, therefore the lattice spacing (d) decreased when the Sr2+ concentration increased. Indeed, the ionic radius of Sr2+ (0.126 nm) is much larger than that of La3+ (0.104 nm) [21]. Hence, when La3+ ions were replaced by Sr2+ ions, charge compensation occurred in the crystalline lattice; consequently, Fe3+ ions with a radius of 0.0645 nm were oxidized to Fe4+ with a smaller radius (0.0585 nm) [20], thus the lattice spacing (d) decreased. Additionally, the characteristic peaks of lanthanum or strontium phases (La(OH)3, La2O3, La2(CO3)3, SrO, or SrCO3) were not observed on the XRD pattern. The absence of diffraction peaks for lanthanum and strontium suggested the alloying of strontium in the crystalline lattice of LaFeO3.
Elemental composition of the samples was determined by energy-dispersive X-ray spectroscopy and atomic absorption spectroscopy, indicating that atomic weight percentage of elements in the samples was equal to their stoichiometric percentages. The calculated errors were from 0.7% to 3.5% for samples synthesized by both methods. This could be attributed to the insufficiency of oxygen when replacing La3+ by Sr2+ in the crystalline lattice.
A study of Lao.8Sr0.2FeO3 and La0.7Sr0.3FeO3 materials by scanning electron microscopy (SEM) showed that the formed crystals had an average particle size of about 80-100 nm but it was demonstrated that the differentiation of particles shapes was more obvious as compared to the pure LaFeO3. This once again confirmed that Sr2+ doping can affect the structure of the LaFeO3 crystalline lattice.
As a result, Sr doping in the LaFeO3 crystal can lead to a change in the lattice structure, particularly in decreased lattice spacing (d) and the alteration of the crystal shape with increased Sr concentration. This can provide a change in magnetic characteristics of doping La1-xSrxFeO3 as compared to pure LaFeO3.
I £
4 + - Lanthanum Iron Oxide a)
850°C
j + ♦ I 1 Li-l
20
30
40
50 60
2-1h?b - Swie
70
so
It
r,
i
A-_
g
950°C
850°C 750°C
20
30
40
50
2-Tb?b -Swle
60
70
so
Fig. 4. XRD pattern of La0.gSr0.1FeO3 sample heated at 850°C (a) and stack XRD diagram of Lao.gSro.iFeO3 heated at 750°C, 850°C and 950°C (b) (t=1 h)
Table 1. The comparison of lattice spacing (d) of La1-xSrxFeO3 (x=0.1; 0.2; 0.3) samples heated at 850°C (t=1 h) (from XRD results, Fig. 5, 6, 7)
No Lattice spacing d; A
Lao.9Sro.iFeO3 Lao.sSro.2FeO3 Lao.zSro.3FeO3
1 3.93402 3.91905 3.91813
2 2.78176 2.77718 2.76904
3 2.26766 2.26596 2.26330
4 1.96576 1.96266 1.95876
5 1.76393 1.75545 1.75545
6 1.60434 1.60262 1.60062
7 1.38814 1.38427 1.38427
8 1.24208 1.24057 1.24020
Fig. 5. XRD pattern of La0.8Sr0 2FeO3 sample heated at 850°C (a) stack XRD diagram of Lao.8Sro.2FeO3 heated at 750°C, 850°C and 950°C (b) (t=1 h)
3.3. Magnetic characteristics of La1-xSrxFeO3 nanomaterial
(x=0.0; 0.1; 0.2; 0.3)
Study of the magnetic properties of nano La1-xSrxFeO3 materials (x = 0.0, 0.1, 0.2, 0.3) calcined at 850°C (t=1 hour) on vibrating sample magnetometers at room temperature indicated that the Sr2+ doping in the LaFeO3 lattice influenced their magnetic properties (Fig. 8 and table 2). Indeed, in a magnetic field of 1273.6 kA/m the magnetic characteristics of the material, such as saturation magnetization (Ms), remanent magnetization (Mr), decreased with x from 0.0 to 0.1, then increased with x from 0.1 to 0.2, then decreased again with x form 0.2 to 0.3. Meanwhile, the value of coercive force (Hc) of all samples was greater than 15.92 kA/m and increased in the order LaFeO3 < Lao.9Sro 1FeO3 < Lao.8Sro.2FeO3 < Lao.7Sro.3FeO3 (where, Hc of Lao.7Sro.3FeO3 was 20 times greater than Hc of pure LaFeO3). This can be explained in that the alloying of Sr2+ in the crystal of pure LaFeO3 increased the non-directionality in shape and crystal lattice of the synthesized materials. Therefore, the synthesized La1-xSrxFeO3 samples are hard magnetic materials (Hc > 7.96 kA/m).
The regular change in Msand Mr values calculated for La1-xSrxFeO3 material with the increasing of Sr2+ concentration in the LaFeO3 crystal lattice was due to structure disordering when replacing La3+ by Sr2+ in the LaFeO3 lattice, which caused the poor orientation of
30
I
.c
20
10
♦-Lanthanum Iron Oxide
a) 850°C
yV-'f-r
4
.- ■.-.--■ V- ■ J 1
20
30
40 50 60
70
so
I
.c
b)
- 95 0°C
1
A 1 J 850°C 1 i K
750°C
..........
20
30
40 50 fiO
70
30
Fig. 6. XRD pattern of Lao.7Sro.3FeO3 sample heated at 850°C (a) and stack XRD diagram of Lao.7Sro.3FeO3 heated 750°C, 850°C and 950°C (b) (t=1 h)
Fig. 7. SEM images of La0.8Sr0.2FeO3 sample (a) and La0.7Sr0.3FeO3 (b) heated at 850oC (t=1 h)
Applied feld.tA. a b Applied BiW. kAAi
Fig. 8. Magnetic hysteresis loop of LaFeO3 (a) and Lao.8Sr0.2FeO3 (b) materials heated at 850°C (t=1 h)
magnetic iron, because of the change in the Fe-O-Fe angle with x from 0.0 to 0.1. With the addition of Sr2+ to the crystal lattice to concentration of 20%, the crystalline structure became stabilized and therefore, the Ms and Mr values increased. However, the further addition of Sr2+ to the x value of 0.3 could lead to the over-replacement of La3+ (trivalent) by Sr2+ (divalent). With respect to the charge compensation, some Fe3+ ions were oxidized to Fe4+ ions (the similar phenomenon was observed in the Bi1-xSrxFeO3 [16]). This resulted in the appearance of the magnetic exchange interaction between Fe3+ and Fe4+ ions through O2- (Fe3+-O2--Fe4+), leading to a change in the magnetic properties of the synthesized materials.
Table 2. Magnetic characteristics of La1-xSrxFeO3 nanomaterial (x=0.0; 0.1;
0.2; 0.3) heated at 850°C (t=1h)
Charateristics LaFeO3 Lao.9Sro.iFeO3 Lao.sSro.2FeO3 Lao.zSro.3FeO3
Remanent
Mr, Am2/kg Magnetization: M at H=0 0.510 0.085 0.527 0.104
Saturation
Ms, Am2/kg Magnetization: maximum M measured 1.974 0.220 1.278 0.572
Coercive Field:
Hc, kA/m Field at which M/H changes sign 16.183 28.490 138.928 336.952
4. Conclusions
Powders of Lai_xSrxFeO3 (x = 0.0, 0.1, 0.2, 0.3) with a particle size of about 100 nm was synthesized from La3+, Sr2+ and Fe3+ cations in heated water by co-precipitation method with ammonia and ammonium carbonate as precipitating agents and a calcination temperature of 750°C (t=1 h). The increase of Sr2+ concentration provided to the increase of coercive force of synthesized La1-xSrxFeO3 material. The obtained La1-xSrxFeO3 nanomaterial showed high coercive force (Hc > 15.92 kA/m), hence it was related to the hard magnetic materials and could be used to produce permanent magnets for motors.
Acknowledgments
This work was supported by the Ministry of Education and Science of the Russian Federation in line with government order for Higher Education Institutions in the field of science for 2014-2016 years (project N 225).
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